1. Field of the Invention
This invention relates to a process and a plant for the gasification of solid fuels, especially coal, via partial oxidation at temperature above the respective ash melting points and pressures between 10 and 200 bar.
2. Discussion of the Prior Art
The principal processes for the gasification of coal which are of interest today are those operating at raised pressure, as it brings certain advantages. The reaction periods are reduced, the production efficiencies per unit time are increased and the combustion is simultaneously increased. Furthermore, the production efficiency (relative to unit volume of the gasification chamber) increases approximately proportionally to the pressure. As the plants in which synthesis gas is usually employed require high pressure, for example the ammonia, Fischer-Tropsch or Oxo plants, a considerable proportion of the compression costs is therefore saved.
When various types of coal, such as bituminous, high ash and caking coal, are to be processed in a single coal gasification plant, then the coal is fed into the gasification reactor in powdered or pulverized form and gasified at a temperature above the respective ash melting point. By this means almost complete gasification and a controlled slag discharge are guaranteed.
When gasifying coal at raised pressure, the powdered or pulverized coal is mixed with water and injected into the gasification reactor (c.f., U.S. Pat. No. 3,607,157). It is characteristic of these coal dust gasification plants that fuel and oxygen are fed in a co-current. With the process described in U.S. Pat. No. 3,607,157 the supply of oxygen and fuel is fed from above via a combined jet.
In order to remove the liquid slag remaining after the pressure gasification in the pressurized reaction chamber, it is dropped into a water bath and thereby granulated. The water-slag suspension is then fed through a lock out of the pressure chamber. The slag carried with the gas is washed out in a scrubber.
The process described in U.S. Pat. No. 3,607,157 operates under optimal gasification conditions and allows a perfect discharge of the slag. However, with this process, the sensible heat present in the synthesis gas is not recovered and is lost for further utilization at the cooling stage, which operates by injecting water into the hot gas in a scrubber. The method mentioned in the above patent is unsatisfactory with regard to heat technology and thus also from the economic viewpoint.
Furthermore, the resulting discharge of the synthesis gas from the reaction chamber, which takes place in the above-named process during the separation of slag and gas at rightangles to the down dip of the slag, leads to blockages in the pipe connected to the gasification reactor. These obstructions are caused by liquid and viscous slag particles, which are swept along with the gas flow and adhere to the walls of the pipe on impact.
It is an object of this invention, therefore, to provide a process for the gasification of all types of coal at raised pressure and simultaneously to utilize the sensible heat present in the resulting synthesis gas to the extent possible.
It has, however, to be taken into account that the utilization of the heat via steam generation in a waste heat plant, for example in power stations and other gasification plants, cannot be directly applied to the coal gasification described here, which takes place at raised pressure and at very high temperatures. The gasification must be carried out at high temperatures in order to liquefy the slag, yet the required process must also allow for a cooling of the synthesis gas, containing the originally liquid and viscous slag particles. In this context, care must be taken that the pipe system of the waste heat plant does not become blocked by deposits of the liquid or viscous slag particles, which are carried in the gas. Additionally, due to the further development of the pressure gasification process, in which the synthesis gas and all the slag leave the reaction together at the bottom of the gasification reactor, the part of the slag which is in the form of large liquid or viscous agglomerates and can only be solidified by indirect cooling causing unjustifiable expense, must be separated from the gas flow.
One possible solution to the problem is to remix the hot synthesis gas straight after its production with cooled synthesis gas. By this means slag particles in the gas solidify before entering the cooling system and thus do not block the cooling pipes. The disadvantages of this solution lie in the considerable loss of temperature gradient in the cooling plant and the necessary enlargement of the heating surfaces. The reintroduction of large amounts of cooled gases also necessitates the use of a fan operating at raised pressure. Additionally, cyclones or other mechanical separators are required for the discharge of the increased amount of solid slag particles. This is expensive due to the high pressure and, in each case, requires an additional discharge lock.